1. Field of the Invention
This invention relates to a method for making hydrogen storage alloy powder having good hydrogen storage properties. More particularly, the invention relates to a method for making hydrogen storage alloy powder which exhibits good initial activity and ensures high capacity and prolonged life when applied to as a negative electrode of nickel-hydrogen secondary cells or batteries. The invention also relates to an electrode comprising the hydrogen storage alloy powder obtained by the method mentioned above.
2. Description of the Prior Art
After the discovery of hydrogen storage alloys capable of storing and releasing hydrogen, they have been applied not only to hydrogen storage means, but also to cells or batteries. In particular, alkaline secondary batteries have been already put into practice, and hydrogen storage alloys have been improved one by one to impart thereto higher capacity and more prolonged life. However, while intensive studies are continued, on one hand, on further improvements of the performance of lithium ion secondary batteries, which have been commercially sold in recent years, there is, on the other hand, a strong demand of development of a nickel hydrogen secondary battery which has higher capacity and longer life than existing counterparts.
As is well known in the art, hydrogen storage alloys have been predominantly prepared by a casting method and quenching method. It is also known that the properties of hydrogen storage alloys differ from each other depending on the manner of preparation.
In general, a negative electrode used in nickel hydrogen secondary batteries, makes use of hydrogen storage alloy powder obtained by breaking, into pieces, the hydrogen storage alloy prepared by the casting method. This electrode exhibits not only poor initial activity, but also insufficient capacity and life along with a poor shelf life.
Japanese Laid-open Patent Application Nos. 61-176063 and 5-225975 propose an electrode made of alloy powder of the type discussed above, which is further treated with an alkali and/or acid. Such electrode is improved in initial activity and capacity although not satisfactory, but its shelf life and cycle life are considerably degraded. Further, during the course of the surface treatment and in subsequent steps, the surface is inconveniently liable to deactivate. This could be avoided to a significant extent by further treatment of the electrode with a solution containing a specific type of compound, i.e. a highly conjugated unsaturated compound. However, satisfactory results have never been obtained with respect to the electrode life.
On the other hand, Japanese Laid-open Patent Application No. 63-291363 proposes an electrode using hydrogen storage powder, which is obtained by breaking, into pieces, the alloy prepared by the quenching method. This alloy suffers a reduced degree of segregation of elements with the electrode life being improved. However, the alloy surface appreciably undergoes oxidation, and the resultant electrode has initial activity and capacity lower than the case of the casting method, thus not withstanding use in practical application.
Under these circumstances, treatment with an alkali and/or acid has been studied on the hydrogen storage alloy powder obtained by the quenching method. In this case, the life is improved, but the initial activity, capacity and shelf life are not satisfactory from the standpoint of the practical use.